Q3

FORESTS, FALL 2019
Question set THREE
DUE

TO REVIEW, ANSWER TWO OF THESE THREE QUESTIONS ABOUT NICHE RELATIONSHIPS AND NATURAL SELECTION..

1. Wildebeest in the Serengeti of East Africa have a very restricted calving season. All females give birth within a 3 week period. This is a pretty common phenomenon among mammals and birds that breed in dense populations. It has been hypothesized that this is an 'adaptive' mechanism to reduce loss of calves to predators by "saturating" the predator populations briefly (this is similar to the notion that masting in trees saturates seed predators so that some seeds survive...). In other words, having calves at the same time as all the other individuals in the herd increases relative reproductive success (fitness) (compared to individuals not so synchronized). If this is so, selection would favor any increase in heritable tendency tiwards synchronization. What kind of observations and data could you collect to test this selective hypothesis (be clear how these data would allow you to assess predictions of the predator saturation hypothesis and differences in fitness within the wildebeest population)? (Make sure you are clear what the hypothesis is...)

This is essentially the same hypothesis as we talked about early in term as an explanation for oak masting. The main point here is that to TEST the hypothesis in this scenario, you have to focus on predictions it makes about fitness of females (as defined by survival of calves) that give birth under different conditions. Because this hypothesis is grounded in selective arguments, you need to focus on differences in likely survival of calves (fitness contricution) among females giving birth at different times. There should be higher survival rate with respect to predation (less chance of an individual calf being eaten) when there are more calves around (NOTE that what's important is not the number of calves surviving at each time, but the PROPORTION, or chance that an individual will survive). One prediction, then, is that risk of being eaten should decrease during the peak of the calving season. If per capita RATES of mortality due to predation (risk of being eaten per individual) DON'T decrease as number of calves goes up, the hypothesis would have to be reconsidered (you might also expect rates of mortality to go up near the end of the calving season as numbers of newborns tail off). You might also look at TOTAL numbers of calves being eaten per day (or whatever time unit) over the course of the calving season; the hypothesis predicts that this number should tend to level off at some point (as predator population is 'saturated') -- but this is a bit trickier. You could also compare (if you could find them) smaller populations where the saturation would be less effective -- and calf survival SHOULD be lower. A more manipulative experiment might involve (if you could figure out a way to do it) inducing births earlier or later and seeing if they experiemce greater likelihood of predation (as hypoth would predict) -- or just looking at individuals born before the peak). IN GENERAL, need to make sure argument compares individual reproductive success (that makes it a selective hypoth), AND need to be clear on how your study would assess a necessary prediction of the saturation hypoth.

2. Increased movement of people and materials around the world has, in recent decades, led to the introduction and establishment of many non-indigenous species in our area (for example, about 1/3 of the approximately 2100 plant species growing 'wild' in Vermont are the result of human introduction). Introduced species include plants, animals, fungi, and microorganisms (including some that are pests or pathogens). Many conservationists view such introduced organisms as potential environmental threats, but views on this issue differ.
A. Using what you have learned about species interactions (competition and niche theory), suggest two or three ways in which introduced PLANTS might affect regional natural communities and their diversity. Under what what circumstances would you regard introduction of a non-indigenous species as an environmental 'threat' ? Explain your reasoning (you may need to introduce some subjective valuations here; that's okay). (You can think in terms of any taxonomic group or guild of organism -- plants, animals, parasites, predators, etc. -- for purposes of argument/illustration; if your arguments might apply more to some groups than others, say so...)
B. All of the species that live in our area have dispersed here from further south over the last ~14,000 years; before that this area was ice-covered. Consequently, ecological communities have gone through long series of transformations as species expanded ranges following the retreat of the ice (and, later, may well have 'contracted' their range northward). How are these changes similar to or different from the consequences of introductions of species from other parts of the world by human agency?

This could be approached in several ways. For Part A: there are several possible avenues by which introduced plants could affect local communities and their diversity; if they use resources that aren't currently limited/limiting (i.e., occupy an 'empty niche' ) they might simply persist with little consequence except increasing diversity by one species. If they compete strongly with native species, they'll reduce abundance of one or more native populations and increase likelihood of extinctions, thus reducing diversity. They might change environment in other ways (e.g., changing soil properties or casting heavier shade) which could change which other species can coexist with them (potentially reducing diversity). The question about 'environmental threat' is, perhaps, trickier; the main issue here is to distinguish an 'effect' (like those mentioned alread) from a 'threat'. Is loss of native species or reduction in diversity good, bad, or neither? Unless you think ecological communities have their own goals and ambitions and purposes, that is up to you to determine -- it's a human evaluation, and that is important to recognize. Worth noting that some introduced species actually increase NPP/biomass or diversity; are these 'good' introductions?

For B part, again, several possible lines of thought are potentially relevant. One difference may be in the rate of new introductions over time; at least recently numbers of human introductions has been very high, and probably much higher than rates of new species arriving by 'self-introduction' (although this may not be true for birds...). Maybe more important as a 'meaningful' difference, 'natural' introductions through range expansions are likely to involve shorter distances (and/or movements that don't cross 'barriers'), which might mean that the interacting species are more likely to have had a 'coevolutionary history' of some sort.

3. This figure shows the distribution of two species of cat-tails – Typha latifolia and Typha angustifolia – over a range of depths of water. Negative depth means out of (above) the water. (Cat-tails are the dominant plant in the wetlands around the Dickinson Pond; both of these species occur on campus). The upper graph shows situations where both species occur together (in sympatry); the lower graph shows distributions in situations where only one of the two species occurs (allopatric). The vertical axis is a measure of abundance (don’t worry about different values between the graphs; it's the relative abundance of the two species that's of interest here).
Interpret the patterns observed in terms of fundamental and realized niches for the two species, indicating the implied competitive relationships. If the observed differences between the two graphs are a result of interspecific competition, you might hypothesize that competition is for either light or mineral nutrients (since these are perennial wetlands, it’s presumably not about water!). Cat-tails are rooted in the sediments, and presumably obtain mineral nutrients through their roots. Briefly, lay out an experiment to attempt to test these hypotheses. Explain your methods, and what you would expect if the relevant hypothesis is correct.

This is essentially the same scenario as the trout question on previous set, but with plants; range of depths inhabited in allopatry (bottom) would define fundamental niche (at least on the niche space axis defined by water depth). Fundamental niches are largely overlapping, but it looks like T. angustifoliahas a somewhat broader distribution in allopatry, or fundamental niche (could say it's more of a habitat generalist). (NOTE: some of you focused on depths of maximum abundance, which is okay, but the niche is really about the RANGE of conditions/habitat inhabited) In sympatry, there appears to be an asymmetrical competitive partitioning of overlap area in niche space: the generalist T. angustifolia seems to be inferior competitor over much of the area of overlap of fundamental niches (i.e., is excluded from deeper water in sympatry), but T. latifolia doesn't show much change in it's distributional range in sympatry; realized niche not much change for T.l., but substantially reduced for T.a. (which is now confined to deeper water). This is a common pattern; more 'specialized' species are typically better competitors in their range of tolerance than are more generalist species.. The usual assumptions apply; we don't know if a) the wetlands involved in the two graphs are otherwise generally similar, or b) whether other plant species (and competitors) might be influencing distributions differently in the two situations. To test a hypothesis concerning competition between two species you always have two approaches; you can remove/constrain one species and see if the other expands in number/distribution, OR you can change the availability of the putatively limiting resource and see if there's at least some (perhaps temporary) increase in the species thought to be limited by competition (although both would likely grow until the resource becomes limiting again OR some other resource becomes limiting). Here, you'd need to do experiments that would influence availability of light OR mineral nutrients separately from the other (fertlizing might be easy; changing how much light's available to the putatively inferior competitor would be tricky, but you could do it...)

II. ANSWER THESE THREE QUESTIONS ABOUT ECOSYSTEM PROCESSES

4. Ecosystems come in all scales. A compost pile is a decomposer-driven ecosystem, where the energy input is 'extrinsic'; energy comes into the ecosystem in organic matter from 'outside', and becomes available to compost-pile organisms through decomposition (break-down) of that organic matter by decomposer organisms. The compost pile has internal trophic dynamics, food webs, nutrient cycles, etc. A gardener might want their compost pile-ecosystem to break down organic wastes (thus liberating nutrients tied up in organic material to return to the garden) as quickly as possible; thus, they would want the pile to support large populations of decomposers with high rates of metabolism. There are two main groups of decomposers -- fungi and bacteria. Bacterial decay tends to be faster, while a fungus-dominated compost pile works more slowly. In practice, people have long noticed that compost piles too full of some kinds of material become fungus-dominated and very slow to break down fully; these types of materials include, for example, wood-chips and dry tree leaves of some types. Fast, bacterial decay can be sustained when there is a high proportion of green plant material (vegetable scraps, grass clippings, etc.) or animal wastes. Think about this is in terms of ecosystem dynamics (READ THE HINTS below) and
a) come up with a hypothesis for what might drive a 'switch' between fungal and bacterial dominance in the compost-pile ecosystem.
b) Sometimes a compost pile can get TOO active, leading to breakdown of materials so fast that nutrients are lost before the compost is added back to the garden (or even getting so hot it catches fire!). Given your hypothesis offer a suggestion for how you might cool/slow down a compost pile (perhaps by limiting bacterial dominance) -- and why it should work.
HINTS: The energy source for all decomposers is the break-down of chemical bonds between carbon atoms in organic molecules (especially carbohydrates -- which are 'pure' caron/hydrogen/oxygen). But, of course, other nutrients/minerals are required to build organisms and support their function. Wood and dry leaves have a very high carbon concentration (they're made up almost entirely of combinations of carbohydrates). Green plant materials and animal wastes have much higher concentrations of PROTEINS (remember that proteins are made of nitrogen-containing amino acids). CONSIDER that there is a parallel here with the Lake Washington story.
(A small additional question: small, forest streams are often primarily 'decomposer-dominated' systems as well, with very little primary production IN the stream. Yet these streams have complex and 'lively' food chains. What is the most likely source of the energy (organic matter) input that fuels such systems?)

Probably easiest to start with the suggested parallel with the Lake Washington story (I give hints for a reason!). Where an ecosystem 'switches' from one state with one dominant class of organisms to another state with a different dominant type of organisms, it is likely a consequence of changes in relative abundance of resource inputs and a change in limiting resource (this term is key; use it!) driven by competition. There's a shift in resource types involved here, too -- from 'high-carbon' and low-other-nutrient materials (carbohydrates) to materials with higher relative proportions of nitrogen (proteins). If fungi could operate at very low nitrogen, while bacterial decomposers can't, and if bacteria are better competitors for resources when they're not nitrogen limited, then the differences in compost-ecosystem chemistry described might drive a 'state change' much like what happened in the Lake (though the nutrients involved are C(energy):N instead of P:N). (ALSO, note that limiting resource arguments are always about relative amounts - proportions -- not just total amounts...). In brief; more wood chips, etc. in proportion to N-rich materials makes nitrogen limiting, favoring fungi.

The 'richer' (in nitrogen) compost environment that permits bacteria to outcompete fungi would then allow faster overall decay rates (higher metabolic rates overall) and so more heat output from that metabolism (remember that all respiration generates 'waste' heat). 'Forcing' the system back towards nitrogen limitation by adding more low-N material (like woodchips -- or any 'pure' carbohydrate -- sugar will do!) would force back towards poorer, slower, more fungal-dominated system and less heating.

Some suggested water could play a role. It might -- but be careful about how you make the argument. It might be that bacterial decay is more limited by water availability than fungal decay metabolism, and much the same logic would apply. Properly reasoned, there's a viable hypothesis here, too, but you have to propose that there's a change in limiting resource (You're also missing the rather broad hint about proteins and amino acids. Using water to cool down a pile directly could, of course, work if you kept pouring it on -- but as long as it's 'running hot' with bacterial decay dominant, it'll just heat up again as soon as you stop. Turning the pile or aerating it can have complicated effects, turning may cool briefly, but MOSTLY it tends to allow FASTER decay because it brings more oxygen to the decay organisms -- so actually tends to make piles get hotter and work faster...)

The little stream question is really just to think about energy inputs to ecosystems OTHER THAN photosynthesis. Most small streams get most energy in form of vegetation debris/detritus that falls/washes into stream...

5. Many New England forests have been harvested at intervals of 70-80 years for over 200 years. The following graph shows general living biomass trends for such a forest over this time period, with four logging episodes. You can think of this curve as showing the accumulation of NPP -- the excess of GPP over respiration and decay, accumulating as biomass (or as carbon). The sharply descending parts of the curve show removal of biomass (wood) in logging. Describe any other patterns or trends you see over the several cycles of logging and regrowth. Offer a hypothesis explaining the dominant patterns in terms of ecosystem processes; use terms and concepts from ecosystem ecology (e.g., you might need to refer to gross and/or net production, respiration, limiting resources,...). There's an appearance of unsustainability here; offer two possibilities (derived from your hypothesis) for ‘improving’ the situation; how might you change things to keep the biomass available for harvest from becoming less each time?

Pattern: each logging episode (sharp drops in biomass) is followed by re-accumulation of biomass -- when new growth (NPP) exceeds loss of biomass through death and decay -- over several decades. This increase, in at least first cycle, begins to level off, suggesting that, at ecosystem level, respiration (of all organisms) is 'catching up' with gross primary production. However, each successive episode of logging reduces residual biomass to progressively lower levels, and recovering biomass never reaches levels achieved prior to earlier cuts. Logging episodes appear to be getting somewhat closer together. It may be that recovery is somewhat slower at outset in later cycles.
Hypotheses: Could just be that earlier harvesting with continued removal of large amounts of biomass (both plausibly driven by economic interests) mean that regrowth gets a slower start each time (fewer trees left for seed-source, smaller trees removed), so each interval there's less new biomass after a given interval -- and economic returns drive continued intensification of harvest, so residual is less each time -- a destructive feedback. However, the slower regrowth could also be a sign of declining fertililty -- decreasing availabilty of some limiting resource (presumably a mineral nutrient, since CO2 and water and light availability would not decrease -- if anything, wate rand light might increase -- following logging). THis could be due to direct removal (the wood taken contains nutrients that can't be returned to the soil through decay), or by indirect effects of logging (like soil erosion).
How to fix it: If first hypothesis is right, then some combination of simply waiting longer between harvests and taking less wood (not reducing residual biomass to progressively lower levels) might be all that it would take. If there is loss of critical/limiting resources (and there almost has to be some), then the time between harvests would have to be long enough to allow replacement of lost nutrients through accumulation of inputs (from weathering of rock, atmospheric deposition, whatever -- remember the Hubbard Brook story). OR, you could simply add nutrients actively (fertilize) to replace lost limiting resources (which is what we'd think of first in a standard ag system).

6. Human burning of fossil fuels injects large amounts of carbon dioxide into the atmosphere. CO₂ is, of course, the source of carbon for photosynthesis and so an essential resource for primary production (by photosynthetic autotrophs). It has been suggested that added CO₂should, therefore, act as a fertilizer, increasing plant growth and NPP. If this were the case, there is the potential that ecosystems would become 'carbon sinks', removing (some of) the excess CO₂from the atmosphere and sequestering it in added biomass, thus reducing the rate at which this most important 'greenhouse gas' builds up in the atmosphere. This would be a desirable thing. However, as we have seen, the regulation of ecosystem productivity is complex, and this outcome depends on several other things. Describe at least one assumption -- in terms of ecosystem process and properties -- of this hypothesis; that is, what would have to be true before this CO₂fertilization effect (increased NPP resulting in increased sequestration of carbon) could take place? Imagine you were in charge of things; think of some practices you could implement to INCREASE the likelihood that vegetation would take up more CO₂as it became available through increased rates of NPP.

The critical assumption here is simply that CO₂is a LIMITING resource for plant growth; one of the most basic principles of ecosystem ecology is that NPP is limited by whatever essential resource is in most limited supply relative to overall needs. If carbon (carbon dioxide) is NOT limiting (if water, or nitrogen, or whatever is the limiting resource for an ecosystem), then adding more CO ₂ will have little or no effect on NPP and carbon sequestration. It's unlikely that CO₂ is, in fact, limiting in most cases (or, at least, unlikely that it stays limiting for long); because the carbon cycle has an atmospheric/gaseous phase, carbon circulates rapidly and freely, and is rarely limiting for long. One way of permitting more biomass formation, then, would be to increase availability of what IS limiting. Irrigating or fertilizing vegetation may stimulate increased NPP, accumulation of biomass, and increased uptake/sequestration of carbon. However, this is complicated and other things can interact (e.g., decay might increase with higher temperatures as well, causing C to be released more rapidly). You could also convert non-forest vegetation to forest, and this would likely increase carbon sequestration since wood holds a lot of carbon for a long time -- however, this isn't really related to the limiting resource assumption...